Lichens are dual organisms consisting of a fungus and an algae or cyanobacteria in a symbiotic relationship. The fungus provides structure and protection while obtaining nutrients, while the algae or cyanobacteria performs photosynthesis. Lichens grow in various forms and can reproduce sexually through spores or asexually through soredia or isidia. Lichens play an important ecological role in extreme environments and are sensitive indicators of pollution.

1. – Lichens - Fungi in closely integrated
association with green algae and/or blue-
green algae.
– Mycorrhizae - Fungi associated with
exterior (ectotrophic) or interior
(endotrophic) of plant roots
– Endophytes - fungi associated with shoots
of plants

2. lichens - predominant
ground cover in tundra
trees dependent on
mycorrhizae

4. - dual organisms
– Lichens very tough - typically grow in extreme
environments (arctic, hot deserts, bare rock etc.) -
often dominant life form where they occur (8% of
world). Consist of:-
– mycobiont (fungus) - obtains water and minerals,
builds shape (thallus), produces the reproductive
structures.
– Phycobiont (alga) - contributes photosynthate
Probably more a case of balanced parasitism than
symbiosis.

5. • The mycobiont has two principal
roles:
 to protect the photobiont from exposure to intense sunlight
and desiccation
– to absorb mineral nutrients from the underlying surface or
from minute traces of atmospheric contaminants.
• The photobiont also has two roles:
 to synthesise organic nutrients from carbon dioxide
– in the case of cyanobacteria, to produce ammonium (and
then organic nitrogen compounds) from N2 gas, by nitrogen
fixation. In some ecosystems such as desert soils, tundra
heaths, and Douglas-fir forests of the Pacific Northwest of
the USA, lichens can provide the major input of nitrogen
which supports other forms of life.
http://helios.bto.ed.ac.uk/bto/microbes/lichen.htm

6. – Nomenclature - there are ca 20,000 sp.
named after the mycobiont (500 genera)
– Structure - The mycobiont forms the major
part (95%) of the thallus (body) and
produces any sexual spores. The photobiont
exists as cells, often in layers just below the
surface, within the thallus

7. • The Phycobiont.
– Only 24 genera of algae, either Chlorophyta (green
algae) or Cyanobacteria (prokaryotes - blue-green
algae). In 70% of lichens the alga is a sp. of
Trebouxia.
– Some algae (not Trebouxia) can be free living
– some lichens have both blue-green and green algae
as phycobionts in same thallus.

8. Trebouxia cells - the
most common green
algae found in lichens
Cells of Nostoc - a
blue-green alga
found in lichens.

9. –Mycobiont.
–98% Ascomycetes, rest Basidiomycetes.
–40% of all Ascomycetes are obligate lichens.
–About half have teleomorph + anamorph,
half teleomorph only + small number
anamorph only. Never free-living
–apparently originated many times
independently

10. fungus hyphae
algal cells

11. fungal
hyphae
algal cells

12. – Lichens grow in a variety of forms:-
• crustose - thin crusts tightly attached to surfaces
• foliose - leaf like protrusions attached to surface by
rhizines
• fructicose - bush-like branched aerial projections
• squamulose - aerial structures made of small scales
• leprose - loose powdery thallus

13. Crustose
lichens

14. crustose type of
growth

15. crustose type of growth

16. crustose lichen on bark

17. Foliose
lichens

18. Foliose type of growth

19. crustose
foliose
fructicose (reindeer
moss)

20. fruticose type of
growth

21. fructicose
lichens

22. Squamulose type of growth
podetia

23. – fungus can reproduce:-
–sexually by ascospores (no algal component)
- spore must ‘locate’ a suitable alga
–asexually
- soredia - powdery masses of alga + hyphae
- isidia - finger like masses of alga + hyphae

24. • Sexual reproduction.
• Most lichen fungi produce apothecial ascoma
(cup-shaped)
• Asci form in the apothecia and release ascopores -
BUT no algal component.
• A few lichen fungi are basidiomycetes and
produce ‘mushrooms’

25. apothecia

27. – Ascospores contain no algae - so how does the lichen
reassemble ?
– The fungus can grow slowly for a while without alga - grows
as disorganized mass - no shape. Once it meets alga, grows
as organized mass into lichen form.
– needs to find an alga of correct species to survive -
sometimes can last a little longer by forming temporary
associations with other algae
– we can only simulate reassembly in the lab when we starve
both components first.

28. • Asexual reproduction.
– Soredia - algal cells wrapped in threads of fungus
– Isidia - tiny projections from the surface of the lichen
that can break off easily and grow into a new lichen.
Most lichens are very brittle when they're dry, and some
depend on just plain breakage to produce fragments that,
like soredia and isidia, can be blown around by wind,
washed along by water, or carried off as passengers on
insects or birds.

29. Soredia
Isidia

30. Lichens
– lichens produce many unique compounds
– weak phenolic acids
– provide resistance to grazing
– UV resistance?
– Important in sp. identification
– some antibiotics,
– fungal genes produce these compounds
but only active when alga present !

31.  Lichens grow slowly (only 5-10% of mass is
alga and they are often in very dry situations)
but are extremely resistant to effects of
drought, floods, heat or cold.
 Lichens have great longevity - can live for
thousands of years.

32. – Lichens - are a very important ecological
component in extreme habitats
–eg in tundra - - dominant lifeform - provide
grazing (‘reindeer moss’)
–weathering of bare rock ---> soil
– often absent from agricultural or urban
environments (sensitivity to pollution)

33. Lichenometry (studies of lichen
growth rate) used in geology
(how long since rocks emerged
from ice) and pollution.
Lichens have no roots - depend
on rain - so ultrasensitive to rain
borne pollution
Montreal area showing
correlation between SO2
pollution (below) and
lichen presence (red dots
on map at left)

34. Lichens and Pollution.
Lichen fungi are extremely efficient in accumulating
nutrients from trace levels in the atmosphere and so are
especially sensitive to pollution.
An extreme demonstration of this is the ability of lichens to
accumulate radioactive isotopes from the environment.
Following the Chernobyl disaster, the lichens (mainly
Cladonia rangiferina, the "reindeer moss") of northern
Scandinavia accumulated so much radioactivity that reindeer
feeding on them were considered dangerous for human
consumption.

35. Oakmoss lichen -
important in fine
perfumes (S-central
Europe)
Usnea filipendula
traditionally used
as medicine for
wounds - has
been found to
contain some
potent
antibiotics)
Wolf lichen" (Letharia
vulpina) - the most
widely used dye lichen
for native peoples in
North America,

36. Flying squirrel nest
made entirely of
Bryoria
FOOD. Mountain
goats, Caribou eat
lichens.
The golden plover uses
Thamnolia vermicularis in
its nests. More than 50
species of birds in North
America known to use
lichens in nest building.
NESTS
CAMOUFLAGE -
lacewing insects
covered in lichen
FOOD. - Witch's hair
lichen blows off trees -
helps black-tailed deer
survive when other
forage under snow

37. A case of reverse lichenosis or just a universal parasite ?
The large marine brown alga, Ascophyllum nodosum
always contains the fungus, Mycosphaerella ascophylli

38. 2% of lichens have
fungi belonging to the
Basidiomycetes and so
produce mushroom-type
fruiting bodies. e.g.
Omphalina ericetorum
Multiclavula
corynoides

39. Follow-up to Lecture
• Read Chap 7
• View Fifth Kingdom CD/ Web page (Chap 7)
• Review these lecture notes on Web
• know main features of Lichens
• review terms -
• mycobiont; photobiont; soredia; isidia
• 5 growth forms (crustose etc.)